Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain
the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in
Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles
Atomic clouds stabilized to measure dilation of time
Tests of relativity once required accurate clocks separated by thousands of kilometres. Optical techniques have now made such tests possible in an atomic cluster measuring no more than one millimetre in size.
As Albert Einstein predicted in his theory of general relativity, the gravitational field of a massive object distorts space-time, which causes time to move more slowly as one gets closer to the object. This phenomenon is known as gravitational time dilation, and it is measurable — particularly in the vicinity of a very massive object such as Earth. The measurement requires a sufficiently accurate clock, and, today, the most accurate timekeepers are atomic clocks, which keep time by detecting the transition energy between two electronic states in an atom. Bothwell et al.1 and Zheng et al.2 now report astounding progress in the stability of atomic clocks using ensembles of ultracold strontium atoms. Bothwell and colleagues even managed to measure the degree to which time is dilated by gravity — a quantity known as gravitational redshift — in a single atomic cloud.